When a lead-acid battery discharges electricity, sulfuric acid in electrolyte solution is consumed and water is generated. The electrolyte solution is thus reduced in specific gravity. Electrolyte solution contain dilute sulfuric acid and included in a lead-acid battery typically tends to have a higher freezing point at lower specific gravity. The electrolyte solution in the lead-acid battery is occasionally frozen and increased in volume in a cold region. This causes a problem that a corner of a container is likely to crack. In particular, overdischarge (discharge of or exceeding a rated capacity) further decreases the specific gravity and increases the freezing point. The problem described above is serious in this case. Crack at a corner of a container can be possibly suppressed by thickening the corner of the container, for example, as described in JP-A-2009-259450. A container is defined in outer shape by a standard and the container can be thickened only inward in some cases.